A typical cladding-pumped fiber device comprises a single-mode core and a plurality of cladding layers. The inner cladding surrounding the core is typically a silica cladding of large cross-sectional area (as compared to the core) and high numerical aperture (NA). It is usually non-circular (e.g., rectangular or star-shaped) to ensure that the modes of the inner cladding will have good overlap with the core. The outer cladding is commonly composed of a low refractive index polymer. The index of the core is greater than that of the inner cladding which, in turn, is greater than the outer cladding.
A major advantage of the cladding pumped fiber is that it can convert light from low-brightness sources into light of high brightness in a single mode fiber. Light from low brightness sources, such as diode arrays, can be coupled into the inner cladding due to its large cross-sectional area and high NA. In a cladding pumped laser or amplifier the core is doped with a rare earth such as Er. The light in the cladding interacts with the core and is absorbed by the rare-earth dopant. If an optical signal is passed through the pumped core, it will be amplified. Or if optical feedback is provided (as by providing a Bragg grating optical cavity), the cladding-pumped fiber will act as a laser oscillator at the feedback wavelength.
U.S. Pat. No. 5,864,644 to DiGiovanni et al. teaches that light is coupled from a plurality of semiconductor emitters to a cladding-pumped fiber via a tapered fiber bundles fusion spliced to the cladding-pumped fiber. Individual semiconductor broad stripe emitters can be coupled to individual multimode fibers. The individual fibers can be bundled together in a close-packed formation, heated to melting temperature, drawn into a taper and then fusion spliced to the cladding-pumped fiber. The taper is then over-coated with cladding material, such as low index polymer. In addition, a fiber containing a single-mode core can be included in the fiber bundle. This single-mode core can be used to couple light into or out of the single-mode core of the cladding-pumped fiber.
FIGS. 1-5 demonstrate an existing approach to making a side pumped fiber, which is consistent with the description in U.S. Pat. No. 5,864,644. In this example, 9 outer fibers A are combined with a center signal fiber. More specifically, FIG. 1 shows that nine 200/220 μm (0.22 NA) pump fibers A are disposed around a 420 μm mandrel B, which is used as a spacer. The ends of the nine fibers A are polished, using an off-the-shelf polishing machine C, to create a flat face—or fibers having the same length, see FIG. 2.
In FIG. 3, the polished ends of the nine fibers A are spliced to a fused silica capillary tube D. The capillary tube is hollow, and has an inner diameter of 405 μm and an outer diameter of 900 μm. In FIG. 4, the capillary tube is tapered with hydrofluoric acid until the inner and outer diameters are about the same. Prior to this step, a plug can be inserted in the center of the hollow capillary tube D, such that the taper goes from the outer diameter to the inner diameter at the plug E. The capillary tube D can then be used as a combiner.
In FIG. 5, the plug E is removed and a 400 μm signal fiber F is inserted through the hollow center of the capillary tube D, and through the nine outer fibers A. As a result, the nine outer fibers A surround the center signal fiber F. Pump laser light from the outer fibers A is side pumped into the signal fiber F within the capillary tube (or combiner) D. Optically, this can work well. However, for each pump fiber A, two (2) junctions exist at the combiner D, the pump fiber A to the combiner D and the combiner D to the signal fiber F. Each junction experiences some optical loss and generates heat. Since light from nine different source fibers A is pumped into the signal fiber F at the combiner D, there are many junctions at the combiner D, all generating heat. This can cause a hotspot at the combiner D. Also, this approach in not particularly scalable, since only a fixed number of pump fibers A can be used with such combiners D.